Carbon and chlorine kinetic isotope effects and solvent effects on the hydrolysis of chloroformates

Carbon and chlorine kinetic isotope effects and pseudo-first order rate constants were measured for the hydrolysis of ethyl, benzyl, and isopropyl chloroformate in various mixtures of the binary solvent water-1,4 dioxane. Rate constants decreased as the percentage of 1,4-dioxane increased for all three chloroformates. Carbon-13 kinetic isotope effects (k1|2/k13) for hydrolysis of these compounds were 1.039-1.042 for ethyl chloroformate (0-75% 1,4 dioxane), 1.034-1.099 for benzyl chloroformate (25-75% 1,4-dioxane), and 1.025-1.037 for isopropyl chloroformate (15-75% 1,4-dioxane). Chlorine 37 kinetic isotope effects (k35/k37) were measured for benzyl and isopropyl chloroformates in 25% 1,4-dioxane (1.0088, benzyl; 10080 isopropyl) and 75% 1,4-dioxane (1.0090, benzyl; 1.0102, isopropyl). These data are consistent with an associative mechanism for benzyl and ethyl chloroformates, but hydrolysis of isopropyl chloroformate appears to be dissociative in polar solvents. (author)